Further Insight in the High Selectivity of Pb Removal over Cd in Natural and Dealuminated Rich-Clinoptilolite.

This research aims to understand the experimental results on the high selectivity of Pb removal over Cd in natural and dealuminated rich-clinoptilolite. For this purpose, we have considered the results of experimental and Density Functional Theory (DFT)-based simulated annealing (SA) on sorption of Pb and Cd from aqueous solution. The dealumination process of natural clinoptilolite (Nat-CLI) was done by HSO solutions at different concentrations (0.1-1.0 M). The results show that the maximum sorption capacity (q) of Pb and Cd varied from 224.554 × 10 to 53.827 × 10 meq/g, and between 39.044 × 10 to 20.529 × 10 meq/g, respectively, when the values of Si/Al ratio change from 4.36 to 9.50. From a theoretical point of view, the global minimum energies of natural and dealuminated clinoptilolites before and after sorption of Pb and Cd were calculated by an SA method, where heating-cooling cycles were modeled by ab initio Molecular Dynamics followed by energy minimization. The theoretical results confirmed that for all Si/Al ratios, the sorption of Pb and Cd takes place, and for dealuminated systems, the exchange energy outcomes are more favorable for the Pb cations. Since such energy differences are very small, it is not explained from a thermodynamic point of view. On the other hand, it could be understood from a kinetic perspective. In this way, we set that the atomic structural properties of the zeolite modify the first hydration coordination sphere of metal cations.